Flip chip LEDs are desirable in many applications since they do not use wire bonding. Both electrodes are located on a bottom surface of the LED for direct bonding to metal pads on a submount. Bonding may be accomplished by ultrasonic bonding, solder, conductive adhesive, or other means. Light exits the surface of the LED opposite the electrodes.
In a typical LED flip chip, the epitaxial p-type layer is the bottom layer and is contacted by the bottom anode electrode. A portion of the p-type layer and active layer must be etched away to expose the underside of the epitaxial n-type layer for being contacted by the bottom cathode electrode. This etching creates distributed vias through the p-type layer that expose the bottom surface of the n-type layer. The via openings are then insulated, and metal is deposited in the openings for contacting the n-type layer.
Such topography is typically achieved by dry-etch of the semiconductor material in a plasma environment with active ionic species. The presence of such structure within a thin film device requires precise control of the plasma etch. Furthermore, the dry etched epitaxial surface, to be interfaced with contact metal, is sensitive to the damage caused by impingement of energetic ionic species in the plasma. From a thermal and mechanical point of view, the corners of such a step structure are prone to becoming failure initiation centers as a result of stress distribution within the sandwiched films. The film growth over the mesa or via structure typically requires a minimum step coverage, leading to higher equipment cost and the requirement of tighter in-line process control.
Accordingly, it would be desirable to avoid such etching of vias to form the flip chip LEDs.